The present invention relates to a railroad car wheel and, more particularly, to a railroad car wheel of a shape in which a plate portion having a slight inclination toward the outside of the track is formed in a straight line from a boss portion to a rim portion and the rim portion is displaced toward the outside of the track from the boss portion.
Conventional railroad car wheels for export, particularly those for export to the United States are of the type commonly referred to as an A-type wheel in Japan, having a conically shaped plate portion between a boss portion and a rim portion which is displaced outwardly from the boss portion with respect to the center of the track, in which the amount of displacement of said conical plate portion is 30 mm or less, which value is not especially large. In the railroad car wheels of this shape, when some extraordinary brake such as brake force locking is applied and the temperature of the rim tread surface rises to 500.degree. C. or above, a circumferential residual tensile stress is caused in the rim portion after cooling. Experiments show that the more severe the braking conditions, the larger the value of the residual tensile stress.
While fracture of the wheel is sufficiently accounted for by fracture mechanics, in the case where a flaw such as a thermal crack is present in the interior of the rim portion fracture of an wheel may be caused if the extraordinary brake force as described above is applied thereto.
According to fracture mechanics, the condition for fracture is: ##EQU1## where, K.sub.IC : fracture toughness (kgf/mm.sup.3/2)
K.sub.I : stress intensity factor (kgf/mm.sup.3/2) PA1 .eta.: stress intensity magnification factor PA1 .sigma..sub.t : residual circumferential stress (kgf/mm.sup.3/2) PA1 a: depth of thermal crack (mm)
Accordingly, fracture of the wheel is probably caused depending upon the magnitude of the values K.sub.IC, .sigma..sub.t and a. By experimentally obtaining distribution of stress in the material, the approximate value of the stress intensity factor K.sub.I can be calculated with respect to the depth of thermal crack a. In the conventional wheel material, for instance AAR Standard Class B (Carbon content 0.57-0.67 wt %), the fracture toughness K.sub.IC is 150-200 kgf/mm.sup.3/2, which shows, for example, in the case where a drag brake force is applied for one hour, where braking force .mu.P is 300 kgf and speed is 88 km/h, from the result of calculation that wheel fracture probably occurs at a depth of the thermal crack a of 11-26 mm.
In the material of railroad car wheels currently in use, occurrence and growth of a thermal crack, which depends partly upon braking conditions, cannot be avoided in severe conditions. Accordingly, whether or not a wheel fracture will occur depends largely upon the residual stress caused by extraordinary braking.
For preventing such a wheel fracture, while it is effective to increase the fracture toughness of the material, the most effective step for this purpose is to improve the shape of the wheel so that no substantial residual stress is caused by heat due to extraordinary braking force to thereby leave the wheel nearly in compressive residual stress at the time of its manufacture. Even when some residual tensile stress is caused, if its value is small and the stress intensity factor for the conceivable depth of the thermal crack is fairly small compared with the value of the fracture toughness, there will be no problem of fracture of the wheel.